The freeze-thaw technique for exfoliation of graphite: A novel approach for bulk production of scroll-free graphene oxide sheets

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Research Article
2017, 8(3), 262-264
Advanced Materials Letters
The freeze-thaw technique for exfoliation of
graphite: A novel approach for bulk production
of scroll-free graphene oxide sheets
Vishal S. Makadia1*, Lalit M. Manocha2, Satish Manocha3, Hasmukh L. Gajera4
1
R&D Dept., Royal Castor Products Ltd., Siddhpur, 384151, Gujarat, India
DMSRDE, GT Road, Kanpur, Uttar Pradesh 208013, India
3
University School of Basic and Applied Science, GGS Indraprastha University, Delhi, 110078, India
4
Dept. of Phy. Sci., PDPIAS, Charotar University of Science and Technology, Changa , 388421, Gujarat, India
2
*
Corresponding author, Tel: (+91) 8866800517; E-mail: [email protected]
Received: 07 January 2016, Revised: 20 August 2016 and Accepted: 22 November 2016
DOI: 10.5185/amlett.2017.6407
www.vbripress.com/aml
Abstract
A freeze-thaw technique is put forth as a novel approach to exfoliating graphene oxide sheets (GO-sheets) in aqueous media.
This method does not use shear force or high-temperature treatment at any stage. Avoiding these factors prevents scrolling
and promotes defect-free synthesis of the graphitic planes. The research shows how the freeze-thaw technique successfully
exfoliates graphitic planes without producing scrolls or defective graphene oxide planes. Further, when compared to
conventional exfoliation methods, it was found that the freeze-thaw technique increased the surface area significantly.
Copyright © 2017 VBRI Press.
Keywords: Freeze-thaw, graphene oxide, exfoliation, carbon materials, nanoparticles.
Introduction
Graphene and graphene oxide sheets (GO sheets) are 2D
materials having outstanding physical and mechanical
properties [1]. High-tech composite fabrication and
synthesis of high surface area material are emerging as a
major focus of many researchers [2,3]. Previously
reported GO sheet bulk production methods involve the
use of either mechanical or shear force such as
ultrasonication [4] or a rapid increase in temperature
[5,6]. GO sheets are scrolled upon exposure to shear
forces such as ultrasonication or stirring [7,8], ball milling
[9] or incur backbone plane defects upon thermal shock
[5,6]. Scrolling and defects in the original planer shape
reduce the total surface area and homogeneity of
dispersed moiety, and they corrupt the properties of the
GO sheets.
In the present research, a novel approach using freezethaw cycles was employed to separate graphitic planes in
dispersed form. Modified Hummer’s method [10] was
applied to prepare graphite oxide dispersion. These
dispersed graphite oxide particles were exfoliated by
freeze-thaw technique. As described in the research, the
freeze-thaw cycles use the expansion behavior of water
upon freezing and low temperature. No shear force is
employed. The results show that the freeze-thaw
technique successfully avoids scrolling and the insertion
of defects in the material’s backbone honeycomb
structure, as shown in Fig. 1.
Copyright © 2017 VBRI Press
Fig. 1. Exfoliation of graphitic planes by freezing and thawing and
ultrasonication.
Proposed theory of the freeze-thaw process
Water exhibits uncommon freezing behavior; it contracts
up to 4oC and slightly expands between 4oC to 0oC, and at
0oC it significantly expands up to 9% upon freezing and is
converted into ice [11]. This is because of its salient
feature - hydrogen bonding between two molecules and
the molecular rearrangement leads to expansion upon
freezing [12]. Graphene oxide planes dispersed in water
contain water molecules intercalated between two
successive GO planes [13]. During freezing, expansion of
this inter-planer water causes expansion of inter-planer
262
Research Article
2017, 8(3), 262-264
space. Upon thawing, solidified expanded water melts,
and water fills up the increased space by insertion of more
water. Repeated cycles of freezing and thawing will
separate the graphitic layers from each other considerably.
As an exfoliating agent, water is promising because of
its increased viscosity in nano-confinement. Research
shows that the viscosity of water dramatically increases
when water is confined between two hydrophilic planes
with inter-planer space ≤ 1 nm. [14]. During expansion,
the increased viscosity of the water between the planes
prevents the water from coming out of the open space in
the planer stacks which helps to overcome the problem of
exfoliation agent diffusion [6].
Experimental
Graphite flakes were oxidized by the Modified Hummer’s
Method [10]. Special care was taken to avoid scrolling
during oxidation of the graphite flakes as a result of rapid
stirring, ultrasonication, or vigorous fluid rotation. In each
step, shaking with a horizontal bath shaker was
substituted for stirring. The bath shaker frequency was
kept below 80 cycles per minute to avoid tornado-type
fluid rotation. Very slow glass rod stirring was applied
whenever necessary. The final product was termed
graphene oxide slurry (GO slurry). Thick, viscous GO
slurry was diluted with water in 1:9 ratio.
Twenty-five mL of diluted GO slurry was pipetted out
and slowly poured into a one-sided close-ended glass tube
having a two cm inner diameter and a one mm thick glass
wall. The glass tube was immersed in a chiller bath prechilled to -10oC. To ensure freezing of the whole mass,
the GO slurry mixture was chilled to -5oC. The frozen
mass was then removed from the chiller bath and kept in a
water bath at 25oC until the temperature of the GO slurry
reached 15oC. The freezing and thawing cycles were
repeated 10 times. The mixture in the tube was found to
be yellowish-brown, cluster-free, and uniformly
distributed. It was directly analyzed without any
additional treatment such as ultrasonication, stirring, or
vigorous shaking. For comparison, graphitic planes were
also exfoliated by ultrasonication for 20 min in a bath
ultrasonicator, as described in the literature [10].
Advanced Materials Letters
Results and discussion
Oxidized graphite is a planer-stacked group of GO sheets
having oxygen containing functionalities [13]. The
d-spacing between planes was found to be 8.44 Å
corresponding to 2θ=10.47o by the X-ray diffraction
method. The total oxygen-containing functionality in
oxidized graphite was found to be 59% by weight by
Thermo Gravimetric Analysis (carried out from 110 oC to
400oC in an inert atmosphere).
Before exfoliation, large particles of graphite oxide
consisting of a group of planes were observed during
TEM analysis, as shown in Fig. 3(a). After freezing and
thawing treatment, the planes separate out resulting in a
single or a few graphitic plane layers, as shown in
Fig. 3(b). While they seem wrinkled, they are noticeably
scroll free, Fig. 3(c) show that mild ultrasonication results
in significant scroll formation. To ensure disruption of
planer stacking, SAED patterns were observed. The
SAED images in Fig. 3(d) indicate large crystallite
structures. As shown in Fig. 3(e), the SAED image of
graphite oxide exfoliated by freezing and thawing shows a
planer structure with a single layer or only a few layers
that probably are layers caused by folding and wrinkling.
In contrast, the electron diffraction of ultrasonication seen
in Fig. 3(f) shows polycrystalline behavior, probably
because of scrolled planes attached with each scroll
behaving as a separate crystal. Raman spectroscopic
analysis was carried out to ensure exfoliation of planes.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3. TEM images of (a) GO-slurry, (b) exfoliated graphitic planes
and scrolls by ultrasonication, (c) exfoliated graphitic planes by freezing
and thawing, and SAED images of (d) GO slurry, (e) exfoliated graphitic
planes and scrolls by ultrasonication, and (f) exfoliated graphitic planes
by freezing and thawing.
Raman spectroscopy, as shown in Fig. 4 gives a better
illustration about scrolling behavior of both techniques.
Raman spectrum of dispersed graphene sheets after
Fig. 2. Mechanism of exfoliation during freeze-thaw cycles.
Copyright © 2017 VBRI Press
263
Research Article
2017, 8(3), 262-264
freeze-thaw cycles shows strong D band at 1328 cm-1 and
a G band at 1607 cm-1. This G band is shifted from 1582
cm-1 as its actual position in pristine graphite due to
extensive defects or functionality generated by oxidation.
Because of the scroll formation in ultrasonication
technique, the G band shifts at 1595 cm-1 which is
towards its position in the pristine state, probably as a
result of overlapping of the graphene planes due to
scrolling.[15] The ratio of the intensity of the D band and
G band that is ID/IG, was found to be ~ 1, indicating good
exfoliation [16,17].
A very high surface area is a significant feature of
graphene planes. Indeed, scrolling reduces the effective
open surface area. The methylene blue dye probing
method was used to determine the surface area of the
functionalized graphene planes in an aqueous dispersion.
As such, the graphite oxide particles showed a surface
area of 730 m2/g; this is consistent with previous reports
[18]. The surface area of the GO sheets produced by the
freeze-thaw technique ranged from 1900 to 1950 m2/g in
various batches. These results show significant separation
of graphitic layers, while in the conventional method of
ultrasonication the surface area was found to range from
1300 to 1400 m2/g. No significant change was found upon
increased exposure time to ultrasonic waves. The
methylene blue probing study shows that dispersed
graphitic sheets produced by freezing and thawing exhibit
a greater surface area compared to sheets produced by
conventional methods. Probably, the absence of scrolling
in the presented method may provide more surface area
for dye molecules to be absorbed. Indeed, using freezingthawing cycles produces a greater surface area, but it is
less than the theoretical surface area of graphene, which is
2630 m2/g. Practically, this theoretical value cannot be
achieved because of folding and wrinkling of planes.
Advanced Materials Letters
TEM and SAED images show that freezing and thawing
prevents scrolling of planes, which cannot be avoided
using conventional methods. Raman spectra ID/IG ~ 1
signifies complete exfoliation. The methylene blue dye
probing shows good improvement in the surface area.
The freeze-thaw technique can emerge as a potential
method to synthesize scrolling-free and planer defect-free
graphene or GO sheets.
Author’s contributions
Conceived the plan: VM, LM, SM; Performed the experiments:
VM, HG; Data analysis: VM, LM, SM; Wrote the paper: VM.
Authors have no competing financial interests.
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Fig. 4. Raman spectrum of exfoliated graphene oxide by Freeze-thaw
technique in comparison with ultrasonication technique and pristine
graphite.
Conclusion
The freeze-thaw technique presented herein is a novel
approach for the scalable synthesis of graphene and
graphene oxide sheets without scrolling of the graphitic
planes. Major scrolling factors such as shear force were
avoided. Results show good concordance with theory.
Copyright © 2017 VBRI Press
264
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